The separation axioms

Idea

The separation axioms are a list of (originally four, now more) properties of a topological space, all of which are satisfied by metric spaces. They all have to do with saying that two sets (of certain forms) in the space are ‘separated’ from each other in one sense if they are ‘separated’ in a (generally) weaker sense. Often, the axioms can also be interpreted in a broader context, such as in a convergence space or in a locale, or under weaker assumptions, such as those of constructive mathematics and predicative mathematics.

The classical theory

First, we will consider how, for topological spaces in classical mathematics, the separation axioms are about sets' being ‘separated’ as stated above. Throughout, fix a topological space SS.

Notice that sets precisely separated by a function must be separated by a function.

Often FF and GG will be points (identified with their singleton subsets); in that case, one usually says distinct in place of disjoint.

Often FF or GG will be closed sets; notice that disjoint closed sets are automatically separated, while a closed set and a point, if disjoint, are automatically topologically disjoint.

Separation axioms

The classical separation axioms are all statements of the form

When FF is a (point/closed) set and GG is a (point/closed) set, if FF and GG are (separated in some weak sense), then they are (separated in some strong sense).

The axioms with names (at least with known to the authors so far of this article) are summarised in the tables below. When a row or column is missing from a table, either no name is known or the implication follows from the converses mentioned after the separation conditions above in the context of that table; there are two potential tables that are completely blank for the latter reason. When an entry in a table is repeated, that corresponds to a theorem that one separation axiom implies another.

When both sets are points:

Stronger condition ↓\Weaker condition →

Distinct

Topologically distinct

Topologically distinct

T0T_0

Separated

T1T_1

R0R_0

Separated by neighbourhoods

T2T_2

R1R_1

Separated by closed neighbourhoods

T212T_{2\frac{1}{2}}

R112R_{1\frac{1}{2}}

Separated by a function

Completely T2T_2

Completely R1R_1

When one set is a point and the other is closed:

Stronger condition ↓\Weaker condition →

Disjoint

Separated by neighbourhoods

Regular

Separated by closed neighbourhoods

Regular

Separated by a function

Completely regular

When both sets are closed:

Stronger condition ↓\Weaker condition →

Disjoint

Separated by neighbourhoods

Normal

Separated by closed neighbourhoods

Normal

Separated by a function

Normal

Precisely separated by a function

Perfectly normal

When the sets are arbitrary:

Stronger condition ↓\Weaker condition →

Separated

Separated by neighbourhoods

Completely normal

Relations between the axioms

First of all, notice that the T1T_1 condition, that distinct points are separated, is equivalent to the condition that every point is closed. Thus, T1T_1 serves as a linchpin between conditions on points and conditions on closed sets.

Many implications between separation axioms can be seen in the following Hasse diagram:

Here, there are two entries at each node; the one on the right includes the T0T_0 axiom, while the one on the left does not. This diagram shows the separation axioms as a meet sub-semilattice of the lattice of all conditions on topological spaces; for example, you can see, by following the diagram upwards, that any space that is both normal and regular must be R3R_3. And since R3R_3 never appears in the tables above, you can take this as a definition of R3R_3.

Warning: TiT_i for i≥3i \geq 3 has been used in different ways in the past, and perhaps by some schools still. Also, all of the RiR_i terms are rare. It is safest to say, for example, ‘normal Hausdorff’ for T4T_4 and clearer to say, for example, ‘normal regular’ for R3R_3. If you want to avoid the subscript terms entirely, then you can, by doing the above and the following:

The separation conditions that appear in T2T_2 and below, or rather their negations, can be easily phrased in terms of the convergence structure, as follows:

Two points are not distinct if and only if they are equal (of course).

They are topologically indistinguishable (that is, not topologically distinct) if and only if every net (or filter) that converges to one must also converge to the other; it's enough to check the ultrafilters generated by the two points.

They are not separated if and only there exists a net (or proper filter) that converges to both.

So by taking contrapositives, it's easy to generalise T2T_2 and below to convergence spaces. (All of the axioms can be generalised to convergence spaces, since the convergence structure determines the topology, but there are several ways to do so, and it's not clear in general which is best.)

For locales, the axioms at the other end are clearest. Here we want to put everything in terms of open sets, so we simply work with the complements of the closed sets that appear in those axioms. Rather than talk about a closed set FF and a neighbourhood UU of FF, we talk about an open set GG and an open set UU such that G∪UG \cup U is the entire space. Now the axioms at the low end are tricky, although there is a standard answer as far down as T2T_2. (Note that every locale is T0T_0, indeed sober.)

In constructive mathematics, while the classical definitions all make sense, they are never quite what is wanted. For the low axioms, one may use, as with convergence spaces, conditions that are classically the negations of the separation conditions; for the high axioms, one may use the open sets that are classically the complements of the closed sets in the axioms. In the middle axioms, these work together; for example, the condition that a point xx is disjoint from a closed set FF becomes the condition that xx belongs to an open set GG.

Specific examples should be found on the pages for specific separation axioms.

References

The English Wikipedia has a decent article, but since I (Toby Bartels) wrote that too, it's not really an independent source. But you can check the references there!